1. Field of the Invention
The invention relates to temperature compensation for ion-selective electrodes, and is particularly applicable to combination electrodes for potentiometric measurements.
2. Background Information
Ion-selective electrodes are commonly used to measure the activity or concentration of ions in solution. Combination electrodes combine both a measuring electrode and a reference electrode in one body. One very popular form of ion-selective electrode contains filling solutions to provide electrical contact between the solution being measured (the analyte) and the electrodes, as well as to provide a stable reference potential for potentiometric measurements.
When an electrode is immersed in an analyte sample, differences in temperature between the analyte sample and the standard solutions used in calibration can cause significant measurement errors unless temperature compensation is taken into account. In order to achieve fast and accurate measurements, it is necessary to measure the temperature of the sample in a fast and accurate manner so that the effect from temperature change can be corrected. Therefore, it is desirable to have a fast thermal equilibrium between sample solution and thermistor in the probe. The time to reach equilibrium (“the response time for temperature measurement”) thus limits one's ability to make a series of rapid measurements on different analytes.
Generally it is desirable to incorporate the temperature sensing element within the electrode itself. One approach incorporates a temperature-sensing element such as a thermistor in the internal filling solution of the working or measuring electrode. See U.S. Pat. No. 7,290,434 B2, issued Nov. 6, 2007 to Jurgen Ammann et al. and assigned to Mettler-Toledo AG. However, some filling solutions are corrosive, and thus the temperature sensing element can be exposed to chemical attack. Additionally, some solutions often impose rigid sealing requirements, which may make inclusion of temperature-sensing elements in them difficult. Thus, while in theory immersion of the temperature sensing element in the measuring filling solution is desirable, practical considerations may militate against this.
Combination electrodes in which the temperature sensing element is incorporated in the reference electrode are also known. See, e.g., certain of the pH electrodes offered by the Schott Instruments Co. However, the large thermal mass presented by the reference filling solution limits the response-time of the temperature sensing element; additionally, temperature gradients caused by diffusion of heat through the reference solution can lead to errors in the indicated temperature and thus errors in the calculated ion concentration.